Fatty acid esters are conventionally obtained by chemical transesterification of refined vegetable oil, in the presence of a catalyst, in particular an acid or basic catalyst. This route for producing fatty acid esters makes it possible to obtain said esters with good yields; however, it has many drawbacks, linked in particular to the need to eliminate the catalyst used, to the difficult recovery of the glycerol and to the high energy consumption. This collection of methods remains laborious, since it comprises many physicochemical conversion steps, which results in a high cost of the fatty acid esters thus produced. Furthermore, the acid or basic catalysts used are not selective with regard to the type of fatty acid, and transesterify all the fatty acid chains, producing a mixture of esters. However, it is very difficult to separate the various fatty acid esters, which have relatively close physical properties. In addition, this type of method does not make it possible to selectively extract the functionalized, in particular hydroxylated, fatty acid esters from the mixture of fatty acid esters obtained via the transesterification reaction. It so happens that it is desirable to have a fraction enriched with hydroxylated fatty acid esters, containing for example methyl ricinoleate (or methyl 12-hydroxy-cis-9-octa-decenoate), since such a fraction represents the starting point in the production of 11-aminoundecanoic acid, a constituent monomer of RILSAN 11, which is a polyamide with exceptional physical properties, developed by the applicant. During the production of 11-aminoundecanoic acid, methyl ricinoleate is subjected to gas-phase thermal cracking. To this effect, it must contain a minimum amount of glycerides, i.e. tri-, di- and monoglycerides, since these products are very difficult to vaporize, and often decompose before vaporization, which results in a reduction in the selectivity of the cracking. Similarly, the methyl ricinoleate must contain a minimum amount of ricinoleic acid, which is also difficult to vaporize.
In light of the aforementioned, the applicant set itself the objective of finding a method for transesterification of vegetable oils containing ricinoleic acid, which overcomes the abovementioned drawbacks of the known transesterification methods and makes it possible to obtain a fraction rich in ricinoleic acid ester.
Methods for esterification or hydrolysis of triglycerides contained in vegetable oils in the presence of lipases are well known. These enzymes have several advantages: they do not require a cofactor, are often commercially available and have a high activity and a high selectivity, even in nonaqueous systems. Esters of glycerol or triglycerides are their natural substrate. Some of these lipases exhibit positional specificity (or regioselectivity), which allows them to distinguish between the central position (sn-2) and the two external positions of glycerol (sn-1 and sn-3).
However, this specificity can be taken advantage of in the case of the ricinoleic acid (RA) present in castor oil only at the cost of a loss of yield, since the latter is composed predominantly of RA residues (from 85 to 90% by weight), the other fatty acid residues present being in particular those of oleic acid, linoleic acid, stearic acid, palmitic acid and linolenic acid. Ricinoleic acid preferably occupies the external positions, but can also be found in the internal position. Moreover, the literature does not describe any lipase that is specific for RA, in a transesterification reaction. Even the lipase extracted from the castor oil plant, which exhibits a preference for RA during the hydrolysis reaction, shows no typoselectivity with respect thereto.
Other lipases exhibit typoselectivity or specificity with respect to the nature of the fatty acid. It is thus known that the lipase extracted from Geotrichum candidum, a yeast-like fungus, exhibits strong specificity toward cis-9 monounsaturated fatty acids in esterification or hydrolysis reactions. Document DE 41 24 248 describes the use of extracellular lipases extracted from Geotrichum candidum for specifically cleaving oleic acid esters (CH3(CH2)7CH═CH(CH2)7COOH). Document U.S. Pat. No. 5,633,151 describes the use of the same lipase for specifically cleaving erucic acid esters (CH3(CH2)7CH═CH(CH2)11COOH).
The publication by Foglia T. A. et al., Eur. J Lipid Sci. Technol. 102 (2000): 612-617, describes the results of tests aimed at determining the selectivity of certain lipases toward fatty acids of plant origin. The results presented in table 1 show that the Geotrichum candidum lipase is discriminating toward ricinoleic acid (hereinafter denoted by RA), during the castor oil partial hydrolysis reaction carried out at 30° C. for 1 to 4 h with a commercial immobilized lipase. Moreover, the results represented in table 2 show that this enzyme is also discriminating toward RA during the reaction for esterification of a mixture of free fatty acids with 1-butanol. The esterification in the presence of G. candidum results, after 24 h of reaction, in a mixture consisting (amounts by mass) of 12% of fatty acid butyl esters and 88% of free fatty acids, the RA representing 94.5% of said free fatty acids. Said mixture therefore comprises approximately 83% of RA.
The particular behavior of an enzyme with respect to a substrate during a hydrolysis reaction does not make it possible to foresee its behavior toward the same substrate during a transesterification or esterification reaction. The specificity of a lipase varies from one type of reaction to another. The publication by Vaysse L. et al., Enzyme Microb. Technol. 31 (2002): 648-655 shows, through the results represented in tables 1 and 2, that the same lipase can have a different behavior toward the same fatty acid ester or the free fatty acid during the hydrolysis, transesterification and esterification reaction, respectively.
The applicant has carried out studies aimed at establishing the effect of the Geotrichum candidum lipase on the fatty acids present in castor oil, in particular on ricinoleic acid (CH3(CH2)5CH(OH)CH2═CH(CH2)7COOH) during the castor oil transesterification reaction. These studies have shown that, surprisingly, said lipase is discriminating toward RA in transesterification reactions in the presence of a light alcohol.